| Abstract
| - Two-photon laser flash photolysis was used to investigate the wavelength dependence of upper excited statechemistry of Rose Bengal (RB) in order to rationalize differences in RB-sensitized inhibition of acetylcholinesterase (ACE) in red blood cell ghosts, observed using different irradiation modes. The absorption of theRB triplet state, generated using 532-nm irradiation, was monitored as a function of wavelength used forsubsequent excitation to upper triplet levels. Triplet state bleaching, due to formation of radical species, wasobserved using wavelengths greater than ∼580 nm to excite into an upper state, Tn. However, no bleachingwas observed at lower wavelengths, corresponding to higher photon energies, but an increase in emissionintensity from the sample, consistent with S1 − S0 fluorescence, was observed following the second pulse. Amechanism for this wavelength-dependent photochemistry is provided, which considers the relative positionsof upper singlet and triplet states. Reverse intersystem crossing, and hence repopulation of the triplet state,are only observed when the energy of the second photon is sufficient to access a triplet state (Tn+1) of similarenergy to the S2 state. At lower photon energies (higher wavelengths), a lower energy state (Tn) state isaccessed, from which radical formation and bleaching of the triplet state ensues.
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